These motor areas are located within the cingulate sulcus at approximately the same anterior-posterior level as the SMA. There are three cingulate motor areas (dorsal, ventral, and rostral; see
Fig. 9.14B ). Each contains a somatotopic map and contributes to the corticospinal tract. Microstimulation in these areas evokes movement similar to that evoked by motor cortex stimulation, except that, again, higher stimulus intensities are needed. Single-cell recordings during movements have shown that the spontaneous activity of neurons in the cingulate motor areas is related to the preparation and execution of movements.
Connections of the Cortical Motor Areas
The motor areas of the cortex receive input from a number of sources, cortical and subcortical; however, the largest source of synapses in an area is the area itself: specifically, the local intrinsic connections. Moreover, all the motor areas described earlier are bidirectionally connected to each other with high topographic specificity (see
Fig. 9.14C ). For example, the arm regions of the primary motor cortex and the cingulate motor areas project to each other. Ascending pathways relay sensory information to the thalamus. This information can reach the motor cortex directly from the thalamus or indirectly by way of the somatosensory cortex. Both somatosensory information and visual information are conveyed to the motor areas from the posterior parietal cortex. The motor areas of the cortex also receive information through circuits that interconnect them with the other major brain regions involved in motor control: namely, the cerebellum and basal ganglia. These two structures project to distinct parts of the thalamus (the ventral lateral and ventral anterior nuclei), which then project to the cortical motor areas.
The output of the cortical motor areas to the spinal cord and brainstem is conducted through several descending pathways. These pathways include not only direct projections through the corticospinal and corticobulbar tracts (to the cranial nerve nuclei) but also indirect projections to the red nucleus and to various nuclei in the reticular formation. Descending projections from these brainstem sites were reviewed in the section .” Control of head and neck muscles is mediated by projections to the various cranial nerve nuclei. The motor regions also project to the cerebellum and basal ganglia, thus completing neuroanatomical loops with these structures. The major connection to the cerebellum is via the corticopontine projections to the basilar pontine nuclei, which in turn project to the cerebellum. In addition, the cortical motor areas project, mostly via disynaptic pathways that synapse in the midbrain, to the inferior olivary nucleus, another important precerebellar area. The cortical motor regions project directly to the striatum of the basal ganglia. Finally, there are major projections to the thalamus by which the cortex regulates the information that it receives.
Activity of Motor Cortex Cells
The role of individual motor cortex neurons in the control of movement has been extensively investigated in trained monkeys. In these experiments, discharges from a neuron in the primary motor cortex are recorded during the execution of a previously learned simple movement, such as wrist flexion, made immediately in response to a sensory cue (Fig.
9.17 ). Motor cortex neurons were found to change their firing rates before initiation of the movement, and the onset of this change was correlated with the reaction time (i.e., the time from the cue to onset of the movement). Moreover, in this task, the change in firing of motor cortex neurons was often correlated with the contractile force of the muscle that generates the movement and with the rate of change in force rather than with the position of the joint. These findings suggest that these cells are involved in the final stages of planning and executing movements, which is consistent with the hierarchical view of the cortical motor areas.